According to the sources, the capacitive reactance is given by $X_C = 1/(\omega C)$ . In circuit (a), which is connected to a DC source, the frequency is zero ($\omega = 0$), making the capacitive reactance $X_C$ infinite . Consequently, the total impedance $Z_1$ of the circuit becomes infinite. In circuit (b), which is connected to an AC source, the frequency is non-zero, resulting in a finite capacitive reactance and a finite impedance $Z_2$. Therefore, $Z_1$ is greater than $Z_2$.